1. Field of the Invention
The invention relates to the construction of cathode ray indicators and other bright display devices suitable for use under wide ranges of ambient light conditions and, in more particular, concerns a method of constructing a unique combination of optical filter and external face plate device for operation with an otherwise conventional display in such a wide range of light levels.
2. Description of the Prior Art
Display devices such as cathode ray tubes with contrast enhancement filters are known in the prior art, as is the bonding of a face plate to the viewing face of a cathode ray tube. For example, television receivers often include a protective glass face plate which may be bonded to the viewing surface of the picture tube. Normally, this protective glass is either clear or acts as a neutral filter, absorbing substantially equally light over the entire visible spectrum.
In aircraft instruments and other panel displays designed for use in high brightness ambients, a great degree of contrast enhancement is required, leading to the use of more sophisticated optical band pass filters possessing one or more band pass regions.
In general, the prior filters are pre-assembled multiple-layer devices consisting of two transparent or glass-like sheets bonded together with a very thin transparent plastic layer, the multiple layer device being affixed at a later time to the viewing face of the display by an adhesive layer. One or more of the three layers of the original sandwich may be colored with an appropriate dye to provide the desired contrast enhancement filter characteristic. In one form, the prior art comprises a sandwich structure wherein the two glass-like sheets are transparent and the optical filter dyes are dispersed in the thin bonding material contained between the two sheets. The dyes are selected to pass a predetermined spectral portion of the light emitted by the display device, while absorbing upwards of ninety percent of light falling outside of the pass band region. The bonding material is desirably very thin in order to minimize thickness and weight of the filter sandwich. This places severe constraints on the colored bonding layer. The amount of light which passes through successive incremental portions of a given optical filter diminishes rapidly in geometrical progression as the number of such portions is increased in arithmetic progression (Lambert's Law); therefore, the fully transmitted output light level of a filter diminishes exponentially with the thickness of the filter for a constant density of absorbing centers. Specifically, for an optical absorber, the transmitted light intensity I at a given wavelength is given by the relationship: EQU I=I.sub.o e.sup.-.alpha.t
where I.sub.o is the incident intensity, .alpha. is the optical absorption constant at that wavelength, and t is the filter thickness. It may be seen that for a given transmission, .alpha. and t are inversely related. Where t is to be made small, on the order of 0.1 mm. for this type of filter, .alpha. must be made large.
Two difficulties arise. The filter layer must contain a large concentration of dye which may be limited by the medium's ability to absorb the dye whose concentration must be accurately controlled. In addition, the optical transmission of the thin filter layer becomes extremely sensitive to small deviations from the ideal thickness. An alternative structure which substantially reduces this sensitivity is described in the C. D. Lustig, J. B. Thaxter U.S. Pat. No. 3,946,267 for a "Plural Filter System Cooperating With Cathode Ray Display with Lanthanum Host Phosphor Emission in Two Colors", issued Mar. 23, 1976 and assigned to the Sperry Rand Corporation. In this filter, the dye was introduced, instead, into the very much thicker glass layers, by the addition of the dye materials in the molten glass at the time of manufacture. The sensitivity to thickness variations was greatly reduced to about sixty times less for a 6 mm. glass thickness compared to the dyed bonding layer art. The particular two filter plates used by Lustig et al are readily purchased on the market with precise thickness. However, obtaining glass with other specific dye characteristics, concentrations, and thicknesses can be very expensive and time consuming.
While the basic features taught by Lustig et al represent significant advances, the particular mechanical structure shown by way of illustration is found to be not altogether practical under all circumstances, particularly in airborne equipment. In any event, total assembly proves difficult and costly in the factory in view of the precision required to maintain consistently repeatable characteristics. The multiple layer assembly is normally bonded to the viewing face of the cathode ray tube with a bonding agent such as a clear epoxy material. This has been performed by placing a spacer of predetermined thickness between the cathode ray tube face and the filter assembly, taping the resultant permanent parts of the configuration to hold them temporarily in place, removing the spacer while hoping that the tape will constrain the spacing to remain correct, filling the cavity by pouring into it a liquid epoxy, curing the epoxy to form a solid transparent resin, removing the tape as waste material, and then coating the exposed periphery of the resin with a protective light trap finish or element.
While this prior assembly method is capable of producing a useful product if extreme care is exercised, the method is difficult to control and is very labor intensive and therefore expensive. Tolerances are difficult to be repeatedly and reliably met. Furthermore, the resulting display device has proven to be more bulky and heavy then is consistent with goals for airborne apparatus. Additionally, in applications in which a graticule is desired in the combination, substantial parallax may exist between the graticule and the images to be viewed on the phosphor screen of the cathode ray tube.